Appropriate methods for repeated surgical collection of endometrial tissue from monkeys and characterization of cytosol and nuclear estrogen and progesterone receptors have been developed. When estrogen or estrogen + progesterone replacement therapy was given to castrate monkeys, we found that progesterone receptor synthesis was induced by estrogens. Further, in the fertile menstrual cycle, progesterone secretion from the corpus luteum preserved the endometrium, induced a shift toward enhanced nuclear receptors for estrone.